This invention relates to the testing and evaluation of eye wetting, and more particularly to the wetting of contact lenses worn on the eye of a subject.
It is commonly assumed that the wettability of a contact lens is directly related to its comfort, optical qualities, and wearing qualities. The bare eye is normally protected by a tear film having a thickness in the range of 6-7 microns. This tear film is composed primarily of an aqueous saline solution, with lesser components of lipid and of mucin, an assortment of glycoproteins. The fluid also contains debris, such as dust and sloughed epithelial cells. The normal precorneal tear film on the bare eye is quite stable. The aqueous component may be calculated to thin out to the point of break-up by pure evaporative processes over a time interval of 5-10 minutes. The film is refreshed upon blinking, and the normal interblink interval is substantially shorter than 5 minutes, so drying-out is not normally encountered.
By contrast, when a contact lens is placed on the eye, each blink of the lid refreshes a tear film on the outer surface of the contact lens, called the prelens tear film, which is typically less than 1 micron in thickness. Drying of the film to zero thickness typically occurs in under 10 seconds. A dry lens can be uncomfortable sticks to the eye and lids, and may be expected to display inferior optical performance due to scattering from uncovered surface imperfections. In addition, a lens which has been subject to repeated drying cycles is likely to have permanent solid deposits built up thereon, which accelerate subsequent tear film breakup and promote trauma or infection.
For these reasons, "wettability" of contact lenses is deemed desirable. Various wetting traits have been taken by the industry as measures of wettability. A common measure of wettability is the "contact angle" determined by the angle of the tangential plane of the edge of a fluid drop with the plane of the lens surface where they meet. This measurement is generally made at a clean interface of a solid with a given fluid, and the measurement must be performed with some delicacy. It is not clear how a contact angle measurement performed under laboratory conditions correlates with the contact angle of tear fluid on an in vivo contact lens in which the lens has been repeatedly exposed to mucins, lipids and other tear film components. One researcher has reported making in vivo contact angle measurements using drops of distilled water placed on the lens. Applicant is not aware of in vivo contact angle measurements using actual tear fluid.
Other approaches to determining in vivo wettability might involve adding a dye, such as fluorescein, to the tear fluid to visualize the thin prelens tear film. However it is desirable to develop a method or apparatus to measure lens wettability which directly measures the thickness and distribution of the prelens tear film, and which does not introduce extraneous substances, such as dyes or fluids which might alter the measurements.
Accordingly, there is a need for a method and apparatus for directly determining the wettability of a contact lens in a manner which meaningfully relates to the environment in which the lenses are used.